The present invention relates to a system and method for performing a volume based three-dimensional virtual examination, and more particularly relates to a system which offers enhanced visualization and navigation properties.
Colon cancer continues to be a major cause of death throughout the world. Early detection of cancerous growths, which in the human colon initially manifest themselves as polyps, can greatly improve a patient""s chance of recovery. Presently, there are two conventional ways of detecting polyps or other masses in the colon of a patient. The first method is a colonoscopy procedure, which uses a flexible fiber-optic tube called a colonoscope to visually examine the colon by way of physical rectal entry with the scope. The doctor can manipulate the tube to search for any abnormal growths in the colon. The colonoscopy, although reliable, is both relatively costly in money and time, and is an invasive, uncomfortable painful procedure for the patient.
The second detection technique is the use of a barium enema and two-dimensional X-ray imaging of the colon. The barium enema is used to coat the colon with barium, and a two-dimensional X-ray image is taken to capture an image of the colon. However, barium enemas may not always provide a view of the entire colon, require extensive pretreatment and patient manipulation, is often operator-dependent when performing the operation, exposes the patient to excessive radiation and can be less sensitive than a colonoscopy. Due to deficiencies in the conventional practices described above, a more reliable, less intrusive and less expensive way to check the colon for polyps is desirable. A method to examine other human organs, such as the lungs, for masses in a reliable, cost effective way and with less patient discomfort is also desirable.
Two-dimensional (xe2x80x9c2Dxe2x80x9d) visualization of human organs employing currently available medical imaging devices, such as computed tomography and MRI (magnetic resonance imaging), has been widely used for patient diagnosis. Three-dimensional images can be foncd by stacking and interpolating between two-dimensional pictures produced from the scanning machines. Imaging an organ and visualizing its volume in three-dimensional space would be beneficial due to its lack of physical intrusion and the case of data manipulation. However, the exploration of the three-dimensional volume image must be properly performed in order to fully exploit the advantages of virtually viewing an organ from the inside.
When viewing the three dimensional (xe2x80x9c3Dxe2x80x9d) volume virtual image of an environment, a functional model must be used to explore the virtual space. One possible model is a virtual camera which can be used as a point of reference for the viewer to explore the virtual space. Camera control in the context of navigation within a general 3D virtual environment has been previously studied. There are two conventional types of camera control offered for navigation of virtual space. The first gives the operator full control of the camera which allows the operator to manipulate the camera in different positions and orientations to achieve the view desired. The operator will in effect pilot the camera. This allows the operator to explore a particular section of interest while ignoring other sections. However, complete control of a camera in a large domain would be tedious and tiring, and an operator might not view all the important features between the start and finishing point of the exploration.
The second technique of camera control is a planned navigation method, which assigns the camera a predetermined path to take and which cannot be changed by the operator. This is akin to having an engaged xe2x80x9cautopilotxe2x80x9d. This allows the operator to concentrate on the virtual space being viewed, and not have to worry about steering into walls of the environment being examined. However, this second technique does not give the viewer the flexibility to alter the course or investigate an interesting area viewed alone the flight path.
It would be desirable to use a combination of the two navigation techniques described above to realize the advantages of both techniques while minimizing their respective drawbacks. It would be desirable to apply a flexible navigation technique to the examination of human or animal organs which are represented in virtual 3D space in order to perform a non-intrusive painless thorough examination. The desired navigation technique would further allow for a complete examination of a virtual organ in 3D space by an operator allowing flexibility while ensuring a smooth path and complete examination through and around the organ. It would be additionally desirable to be able to display the exploration of the organ in a real time setting by using a technique which minimizes the computations necessary for viewing the organ. The desired technique should also be equally applicable to exploring any virtual object.
It is another object of the invention to assign opacity coefficients to each volume element in the representation in order to make particular volume elements transparent or translucent to varying degrees in order to customize the visualization of the portion of the object being viewed. A section of the object can also be composited using the opacity coefficients.
The invention generates a three-dimensional visualization image of an object such as a human organ using volume visualization techniques and explores the virtual image using a guided navigation system which allows the operator to travel along a predefined flight path and to adjust both the position and viewing angle to a particular portion of interest in the image away from the predefined path in order to identify polyps, cysts or other abnormal features in the organ.
In accordance with a navigation method for virtual examination, a fly-path through a virtual organ, such as a colon lumen, is generated. From the volume element representation of the colon lumen, volume shrinking from the wall of the virtual colon lumen is used to generate a compressed colon lumen data set. From the compressed colon lumen data set, a minimum distance path is generated between endpoints of the virtual colon lumen. Control points are then extracted along the minimum distance path along the length of the virtual colon lumen. The control points are then centered within the virtual colon lumen. Finally, a line is interpolated between the centered control points to define the final navigation fly-path.
In the above method for generating a fly-path, the step of volume shrinking can include the steps of representing the colon lumen as a plural stack of image data; applying a discrete wavelet transformation to the image data to generate a plurality of sub-data sets with components at a plurality of frequencies; and then selecting the lowest frequency components of the sub-data sets.
Another method for generating a fly-path through a virtual colon lumen during virtual colonoscopy includes the step of partitioning the virtual colon lumen into a number of segments. A point is selected within each segment and the points are centered with respect to a wall of the virtual colon lumen. The centered control points are then connected to establish the fly path.
A method for perfonring examination of a virtual colon lumen includes a volume rendering operation. For each view point within the colon lumen, rays are cast from the view point through each image pixel. The shortest distance from the view point to a wall of the colon lumen is detemnined for each ray. If the distance exceeds a predetermined sampling interval, the processing effects a jump along the ray by the distance and assigns a value based on an open space transfer function to the points along the ray over the jumped distance. If the distance does not exceed the sampling interval, then the current points are sampled and displayable properties are determined according to a transfer function.
The methods of imaging and volume rendering also lend themselves to a method of performing virtual biopsy of a region, such as a colon wall or suspected mass. From a volume representation of a region which is derived from imaging scanner data, volume rendering is applied using an initial transfer function to the region for navigating the colon lumen and viewing the surface of the region. When a suspicious area is detected, dynamic alteration of the transfer function allows an operator, such as a physicians to selectively alter the opacity of the region and the composited information being viewed. This allows three dimensional viewing of interior structures of suspicious areas, such as polyps.
In yet another method in accordance with the present invention, polyps located on the surface of a region undergoing examination can be detected automatically.
The colon lumen is represented by a plurality of volume units. The surface of the colon lumen is further represented as a continuously second differentiable surface where each surface volume unit has an associated Gauss curvature. The Gauss curvatures can be searched and evaluated automatically for local features which deviate from the regional trend. Those local features corresponding to convex hill-like protrusions from the surface of the colon wall are then classified as polyps for further examination.
In a further method in accordance with the present virtual imaging invention, a method of performing virtual colonoscopy includes the step of acquiring an image data set of a region, including the colon, and converting the image data to volume units. Those volume units representing a wall of the colon lumen are identified and a fly-path path for navigating through the colon lumen is established. At least one transfer function is then used to map color and opacity coefficients to the wall of the colon lumen. The colon can then be displayed along the fly-path in accordance with the assigned transfer functions.
In the method of virtual colonoscopy, the step of generating a fly-path can include using volume shrinking from the wall of the virtual colon lumen to generate a reduced data set. From the reduced data set, a minimum distance path between endpoints of the virtual colon lumen is generated. Control points along a length of the virtual colon lumen can then be assigned alone the minimum distance path. The control points within the virtual colon lumen are then centered and a line connecting the centered control points is interpolated to complete the navigation fly-path.
Alternatively, a fly-path can be generated by partitioning the virtual colon lumen into a plurality of segments; selecting a point within each segment; centering each point with respect to the wall of the virtual colon lumen; and connecting the centered points to establish the fly-path.
A system for three dimensional imaging, navigation and examination of a region in accordance with the present invention includes an imaging scanner, such as an MRI or CT scanner, for acquiring image data. A processor converts the image data into a plurality of volume elements forming a volume element data set. The processor also performs the further steps of: identifying those volume units representing a wall of the colon lumen; establishing a fly-path for navigating through said colon lumen; and applying at least one transfer function to map color and opacities to the wall of the colon lumen. A display unit is operatively coupled to the processor for displaying a representation of the region in accordance with the fly-path and the at least one transfer function.
An alternate computer-based system for virtual examination, formed in accordance with an embodiment of the present invention, is based on a bus structure architecture. A scanner interface board is coupled to the bus structure and provides data from an imaging scanner to the bus. Main memory is provided which is also coupled to the bus. A volume rendering board having locally resident volume rendering memory recieves at least a portion of the data from the imaging scanner and stores this data in the volume rendering memory during a volume rendering operation. A graphics board is coupled to the bus structure and to a display device for displaying images from the system. A processor is operatively coupled to the bus structure and is responsive to the data from the imaging scanner. The processor converts the data from the imaging scanner into a volume element representation, stores the volume element representation in main memory, partitions the volume element representation into image slices, and transfers the volume element partitions to the volume rendering board.